Usually, a brushless motor needs a detector for detecting the position of magnetic poles of a rotator of the motor. However, for example, in the case where the brushless motor is used in a compressor for an air conditioner, it is not possible to use such a rotator pole position detector since the reliability of the detector cannot be ensured under the condition of high temperature and high pressure. Accordingly, in such applications is employed a method which uses no pole position detector and in which a voltage signal induced in an armature winding is detected and a commutation signal of the motor is generated on the basis of the detected voltage signal.
However, since the voltage signal is induced in the armature winding only in the case where the rotator is rotating, no pole position information can be obtained in the case where the rotator is stopped. In other words, the voltage signal is not available when the motor starts rotation. Accordingly, a specified signal for causing a rotating magnetic field to generate in the armature winding irrespective of the pole position of the rotator is applied upon starting to forcibly rotate the rotator.
Thereafter, the rotator starts to rotate and a voltage signal induced in the armature widing is detected, thereby making it possible to generate a commutation signal of the motor on the basis of the detected voltage signal. Thus, the brushless motor is driven in the method in which the motor is activated in the synchronized operation mode and is thereafter changed over to the rotator position detection operation mode. The adjustment of a voltage supplied to the motor is made using a so-called pulse width modulation scheme in which a voltage is controlled by controlling a duty ratio of a voltage signal pulse with a DC power source voltage being constant.
Various methods have been proposed for the change-over between the synchronized operation mode and the rotator position detection operation mode. In many cases, however, it is difficult to make the switching while a stable condition of operation being kept and a load is imposed on motor driving semiconductor switching elements due to the generation of a mechanical vibration or instantaneous increase of a driving current.